The invention relates to an LED module comprising an LED (light-emitting diode) and a rotationally symmetrical, bowl-shaped collimator lens which is provided with a recess in which the LED is situated, and which collimator lens is also provided with a flat surface from which light generated by the LED emerges, the normal to the surface extending substantially parallel to the axis of symmetry of the lens. The invention also relates to a luminaire provided with a number of said LED modules.
An LED module of the type mentioned in the opening paragraph is known per se. For example in the English-language abstract of Japanese patent application JP 61-147.585, a description is given of such an LED module. This LED module comprises an LED which is secured onto a substrate and which is positioned in the recess of a bowl-shaped collimator lens. This lens is rotationally symmetrical in shape and has an associated axis of symmetry. The position of the LED and the shape of the lens are attuned to each other in such a manner that a large part of the light generated by the LED is converted via refraction and reflection into a parallel light beam which leaves the lens via a flat surface. The lens and the substrate are secured in a metal housing.
This known LED module has an important drawback. The emerging light leaves the lens in a direction that is substantially parallel to the axis of symmetry of the lens. Under certain conditions it is desirable for the parallel beam to leave the lens at a certain angle, viewed relative to the axis of symmetry.
Copending application Ser. No. 09/415,833 filed Oct. 12, 1999 of Keuper and Pashley, the disclosure of which is incorporated herein by this reference thereto, aims at providing an LED module of the above-mentioned type, in which the light emerges at a specific angle relative to the axis of symmetry of the lens, the proposed LED module is additionally compact, the said LED module has a simple structure, and its manufacture is inexpensive. The LED module comprises a LED and a rotationally symmetrical, bowl-shaped collimator lens which is provided with a recess in which the LED is situated and which collimator lens is also provided with a flat surface from which light generated by the LED emerges, the normal to the surface extending substantially parallel to the axis of symmetry of the lens, which LED module in accordance with the invention is characterized in that the surface is provided with a sawtooth-like structure for deflecting the emerging light.
The invention in said co-pending application is based on the recognition that such a sawtooth-like structure offers a good solution to the deflection of the parallel beam leaving the lens. Since the maximum dimensions of the teeth are very small (below 1 mm), such a sawtooth-like structure can be provided in a relatively thin layer (thickness below 1 mm). By virtue thereof, also the maximum dimensions of the LED module remain limited. In comparison to alternative solutions in which a separate prism is arranged in front of the emergent face of the lens, or the lens is beveled, said co-pending application offers compact, inexpensive LED modules which can be readily manufactured. Such a collimator is referred to as the xe2x80x9cflat top tulipxe2x80x9d collimator. In its preferred embodiments, it is preferably a solid plastic piece with an air-filled indentation at the entrance aperture. The wall of the indentation is a section of a circular cone (of cone angle typically about 2xc2x0), and the indentation terminates in a lens shape. The LED (in an appropriate package) injects its light into the entrance aperture indentation, and that light follows one of two general paths. On one path it impinges on the inner (conic) wall of the solid collimator where it is refracted to the outer wall and subsequently reflected (typically by TIR) to the exit aperture. On the other path, it impinges on the refractive lens structure, and is then refracted towards the exit aperture. This is illustrated schematically in FIGS. 1A and 1B. The collimator is designed to produce perfectly collimated light from an ideal point source at the focus. When it is used with a real extended source of appreciable surface area (such as an LED chip), the collimation is incomplete (nor can it ever be complete for any design, for functional reasons). Instead it is directed into a diverging conic beam of cone angle xcex8. Improving (i.e. reducing) this angle xcex8 is a primary purpose of the present invention.
This invention represents an improvement in the performance of the Flat top Tulip Collimator disclosed and claimed in our co-pending application Ser. No. 09/415,833 referred to above in terms of reduced size, beam divergence, beam uniformity, and to some degree efficiency. Moreover, it allows a wider variety of choices in optimizing various performance characteristics at the expense of others. This in turn allows more flexibility in the design process, leading to improvements in the various applications that employ the LEDs and collimation optics.
According to the invention, the various optical surfaces of the collimation optics are defined according to one or more constructive rules in order to reduce the angular errors, which result from the finite (i.e. non-infinitesimal) LED source size.
The advantages of the present invention are described in the description that follows in terms of five main characteristics: (1) the beam divergence angle, (2) the exit aperture diameter de, (3) the overall height h, (4) the spatial uniformity of the exit beam, and (5) the efficiency xcex7. Since xcex8 and de are ultimately related by a fundamental theoremxe2x80x94the conservation of etendue, the initial description is geared to one fixed value of de, namely 30 mm. This is arbitrary and for illustrative convenience only, and in no way limits the generality of possible design choices. In the discussion that follows, performance will be described in relation to a 30 mm flat top tulip collimator with a 1 mm square LED chip in the standard LED package with a 2.75 mm radius hemispheric dome lens. This will be referred to hereinafter as the xe2x80x9cReference Collimator 20xe2x80x9d and is illustrated in FIG. 1A. Additionally, we take as the xe2x80x9cbeam divergence anglexe2x80x9d the cone half-angle that envelopes 90% of the beam, and we refer to this angle hereinafter as angle xcex890%.
An object of this invention is to gain improvements in one or more desirable characteristics of a flat top tulip collimator without sacrificing performance in other areas. Such areas for improvement are discussed herein in terms of several distinct applications including high-performance white light illumination, Projection Displays, and Light Generators for signing and illumination.
For high performance white light illumination, especially involving the color mixing of red, green, and blue (xe2x80x9cRGBxe2x80x9d LEDs), all of the above characteristics are important. In particular, for the deterministic color-mixing schemes such as the Beam Splitter Mixer as claimed in our U.S. Pat. No. 6,139,166 issued Oct. 31, 2000 and assigned to the same assignee as this application, or dichroic mixing, the sine qua non is the beam uniformity. This is the case because in these schemes, the various RGB beams are superposed, and if those beams are not uniform (e.g. if they contain imaging information) then that will result in unacceptable color error. Specifically, the lens of the Reference Collimator forms an image of the actual LED chip, which in addition to being generally square has bond-wire attachments, contact electrodes, and possibly other undesirable spatial non-uniformities- and thus directly leads to poor performance in such applications. Adding a diffusive surface property to the lens can disrupt the imaging, but in the Reference Collimator, this also increases xcex890%, which is undesirable.
Another object of the invention is to modify the flat top tulip collimator such that the beam divergence angle of the (smooth) lens portion is less than that of the reflector portion while keeping the reflector divergence low. This will permit using a diffusive surface on the lens to destroy the imaging, while keeping the total divergence low.
For applications such as Projection Video, the absolute quantity of light that can be gotten into a fixed-etendue target is a very important parameter, but also the light must be uniform. Such systems typically require some sort of spatial homogenization optics (i.e. an xe2x80x9cintegratorxe2x80x9d), but it is still desirable that the initial beam be of good uniformity. In many (but by no means all) applications, there is a big premium on keeping the optical path as compact as possible.
It is therefore another object of the invention to provide a means for prioritizing and optimizing among these and other important parameters such that a Projection Video application can achieve one or more performance advantages over such a reference Collimator.
For applications such as Light Generators for signage or for illumination, the requirements are similar to those for Projection Video, except that the beam uniformity is less critical, because the fiber or light pipe that receives the light will substantially homogenize the beam. Thus optimal collimation and total light throughput can be stressed at the expense of uniformity (imaging) problems. Facilitating this specific optimization is yet another object of this invention.
These and other objects of the invention are achieved by a LED module comprising a LED and a rotationally symmetrical, bowl-shaped collimator lens which comprises an inner refractive wall, an outer reflective wall, a first surface having an entrance aperture with a recess in which the LED is situated and which collimator lens is also provided with a second surface (which may be planar or curved) from which light generated by the LED emerges, and, optionally, wherein the normal to the surface extends substantially parallel to the axis of symmetry of the lens, which LED module in accordance with the invention is characterized in having one or more of the following characteristics:
(1) a conic wall portion of the recess of the inner refractive wall at the entrance aperture is modified to include a curved portion, and the outer reflective wall is reconfigured in accordance with said modification of said inner refractive wall to achieve substantial collimation of a source of light at the entrance aperture; and/or
(2) the first surface of the lens is recessed away from the LED source; and/or
(3) the refractive function of said refractive wall (the lens) is divided between two curved surfaces of said lens.
In a preferred embodiment of the invention, the modification of the conic wall portion of the recess of the inner refractive wall at the entrance aperture to include a curved portion is achieved in a structure wherein a portion of the inner refractive wall is at a first angle from the optic axis that is larger than is a second angle from the optic axis of the comparable portion of the refractive wall in a Reference Collimator. For example, the distance d2 from the center of the LED chip to the inner refractive wall at an angle of 45xc2x0 to the optic axis (z axis) is greater for the preferred embodiment of FIG. 2 than the comparable distance d1 in a Reference Collimator. This reduces the beam divergence due to the extended (i.e., non-infinitesimal) size of the LED chip.
In another preferred embodiment of the invention, the modification of the conic wall portion of the recess of the inner refractive wall at the entrance aperture to include a curved portion is achieved in a structure wherein the slope angle of the tangent to the portion of the inner refractive wall at an angle of 90xc2x0 to the optic axis is a distance d3 that is farther away from the vertical than is the distance d4 in the comparable portion of the refractive wall in a Reference Collimator. This increases the steepness of the initial refraction (i.e., for rays at 90xc2x0 to the optic axis), and therefore the steepness of the curve of the outer reflective wall. This-in turn allows for a more compact (and therefore more etendue-efficient) design.
In an especially preferred embodiment of the invention, the surface of the inner refractive wall is specified using a Constant Magnification Prescription whereby the refractive wall and the reflective wall are such that equal intervals of the polar-angle direction cosine in the input beam are mapped onto equal intervals of radial distance from the axis in the output beam. The surfaces are defined point-by-point and the inner wall surface tapers inward towards the optic axis as the polar angle decreases, may inflect and flair away from the optic axis.
In another embodiment of the invention, the modification of the conic wall portion of the recess of the inner refractive wall at the entrance aperture to include a curved portion is achieved in a structure so configured that the average angle of incidence on the inner refractive wall surface has a value close to normal incidence.
In another embodiment of the invention, the first surface of the lens is recessed away from the LED source in a structure wherein a starting point of the lens surface is shifted along a line from the focus point so that an edge ray from the LED source has a direct line-of-sight to the lens surface.
In another embodiment of the invention, the lens is divided between a first surface and a second surface in which the first surface is that portion of a bounding surface of the recess that lies within a cone of a cone angle lens 0 about the optic axis with vertex at the LED center. The second surface may be a planar top surface, or a curved top surface.
The lens may also comprise a diffusing element on or within the lens (using techniques to provide the diffusing element that are well known in the art) to improve uniformity.
For the flat-top variants of the present inventions, the LED module may comprise a sawtooth-like structure provided in a separate foil, which is secured to the second surface of the collimator lens. Additionally, the sawtooth structure when present may be pressed into the surface of the lens or such a surface may be obtained via a replica technique as disclosed in said copending application referred to above. In this technique, a solution of thermally curable or UV-curable material is provided on the flat surface of the known lens. Subsequently, a template whose active surface is provided with the negative of the intended sawtooth-like structure is pressed into this liquid, after which the liquid is cured by exposure to heat or to UV radiation. Subsequently, the template is removed. This method also enables the entire lens in certain embodiments of the invention to be manufactured in a single step. It has been found that particularly (methyl) methacrylate compounds are very suitable for manufacturing the intended collimator lenses.
The invention also relates to a luminaire comprising a box-like housing which accommodates a number of LED modules, each module including a LED and a collimator lens having a structure according to this invention.
In principle, it is possible to use collimator lenses of moldable glass. In practice, however, it is more convenient to provide collimator lenses of a thermoplastic synthetic resin by injection molding. Very good results are achieved using lenses of polycarbonate, (methyl) methacrylate compounds, and the like for manufacturing the intended collimator lenses.